Interpretive Summary: Manure water-extractable P (WEP) data are used in indexes and models to assess P transport in runoff. Methods to estimate WEP vary, without understanding of the effect on how much P is extracted. We extracted WEP from dairy, swine, and poultry manures to assess the effect of single and sequential extractions, drying manures, solution to solid (cm3 g-1) extraction ratios, and method used to analyze P in solutions. We found little difference in WEP extracted by single or sequential extractions. Increases in water : manure extraction ratio from 10:1 to 250:1 increased WEP, but in a decreasing manner so that WEP measurements at ratios of 200:1 and 250:1 were not significantly different. Increases in WEP with extraction ratio varied with manure type, presence of bedding material, and drying treatment. We analyzed water extracts before and after digestion, to examine the effect of P determination by Inductively Coupled Plasma (ICP) spectroscopy on WEP. Digested extracts always contained more P. For manures with bedding, drying decreased the difference in P measured before and after digestion. The opposite was true for manures without bedding. Overall, significant effects of methodology on manure WEP show a need for a standard water extraction method. We suggest using fresh manures, with a solution to solids extraction ratio of at least 200:1, and specifying the method of P analysis in reporting results.

Technical Abstract:
Manure water-extractable P (WEP) data are used in indexes and models to assess the risk of P transport in runoff. Methods to estimate manure WEP vary widely, without understanding of the effect on how much P is extracted. We conducted water extractions on five dairy, swine, and poultry manures to assess the effect of single and sequential extractions, drying manures, solution to solid (cm3 g-1) extraction ratios, and method used to analyze P in solutions. We found little difference in WEP extracted by single or sequential extractions. Increases in extraction ratio from 10:1 to 250:1 increased WEP, but in a decreasing manner so that WEP measured at ratios of 200:1 and 250:1 was not significantly different. Patterns of increased WEP with extraction ratio varied with manure type, presence of bedding material, and drying treatment. Fresh and air-dried manures had similar patterns, which were different from oven-dried manures. The differential effect of oven drying on WEP was greatest for dairy, least for swine, and intermediate for poultry manures. We analyzed water extracts colorimetrically before and after digestion, to examine the potential effect of P determination by Inductively Coupled Plasma (ICP) spectroscopy on WEP. Digested extracts always contained more P. For manures with bedding, drying decreased the difference in P measured before and after digestion. The opposite was true for manures without bedding. Overall, significant effects of methodology on manure WEP show a need for a standard water extraction method. We suggest using fresh manures, with a solution to solids extraction ratio of at least 200:1, and specifying the method of P analysis in reporting results.